G Ras made a suprising cameo appearance in the forums to post this gem. Apparently, some scientists (or, at least the people who answer questions on New Scientist.com without providing any credentials) think they have an equation for predicting poop size.
You can't really argue with this equation. But you really can't prove it either -- to find the individually-variable constants, you'd have to measure the in and out of a person's bowels, and predict from there. And that's kinda circular -- you can only use this equation to predict bowel movement size in people who you've analyzed to find the patterns of their bowel movement size.
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Faeces weight = Wf(1 + Hf) + Wb (1 + Hb) + Wm(1 + Hm)
Where Wf, Wb and Wm are respectively the dry weights of fibre remaining after fermentation in the colon, bacteria present in the faeces and osmotically active metabolites and other substances in the colon which could reduce the amount of free water absorbed, and Hf, Hb and Hm denote their respective water-holding capacities.